GpuWork: Delay GpuWork thread initialization

On boot, `GpuWork::initialize()` tries to register an atom pull
callback with statsd via `setPullAtomCallback()`.

If `setPullAtomCallback()` is called before statsd is ready to
register callbacks, statsd will just drop any requests without
returning an error. The caller has no way to know if the
callback registration failed or should be retried.

This prevents gpuservice from exporting any gpuwork data,
as gpuservice starts with class core, ie quite early upon boot.

Add a delay to GpuWork::initialize() to allow for statsd/bpf init.

Flag: EXEMPT bugfix
Bug: 347657694
Test: statsd_testdrive 10147
Change-Id: Ic5ef399d8a780b082e52f51108a15eecf89f077e
Signed-off-by: Varad Gautam <varadgautam@google.com>